In Firefox, right click on the above link and click "Save Link As..."
Choose a location to save it to (such as "My Documents")
In the Firefox Menu Bar (Top of the screen, File, Edit, View, History, etc.), click Tools, then click Add-ons
Drag the NS_brick.jar file from Windows Explorer to the Themes section of the Add-ons window.
It should be pretty self-explanatory from here on out!